The Obama administration encountered bipartisan skepticism Wednesday as it sought to sell Congress on its nuclear deal with Iran and ward off new sanctions.

In a classified briefing led by Under Secretary of State for Political Affairs Wendy Sherman and open to all House members, officials emphasized that sanctions relief won’t happen until Iran begins to uphold its end of the bargain — a scaleback of its nuclear program, several lawmakers said. The administration also reiterated that the deal is reversible without follow-through from Iran.

“Iran hasn’t taken steps yet to implement what it is supposed to do so we have not taken any steps,” said Rep. Rick Larsen (D-Wash.). “We shouldn’t step first. The world isn’t concerned about sanctions. The world is concerned about Iran’s nuclear program.”

Iranian Ambassador to the International Atomic Energy Agency Reza Najafi estimates the Middle Eastern nation would begin implementation in late December or early January.

The briefing is part of a fresh wave of outreach from President Barack Obama and his top brass toward members of both parties that are itching to pass a new round of sanctions, as the House already did this summer to limit Iran’s fuel exports. The Senate has not yet acted on that legislation and has delayed considering a sanctions bill at the urging of Obama and Secretary of State John Kerry.

Despite nearly two hours of questioning from lawmakers, concerns linger among both Democrats and Republicans, some of whom said the briefing consisted mostly of information already available in the press. The continuation of the Iranian nuclear enrichment program under the six-month deal remains the chief point of contention for Senate Democrats and House Republicans.

“We have essentially acquiesced to allow Iran to enrich uranium,” said Rep. Trent Franks (R-Ariz.), who has authored a bill that calls for a permanent halt for nuclear enrichment as a condition for negotiations. “The level of naivety that Barack Obama has shown regarding Iran’s pursuit of a nuclear weapons capability beggars my ability to articulate.”

“There’s skepticism with everything we do that’s controversial,” said Dutch Ruppersberger of Maryland, the top Democrat on the House Intelligence Committee who generally supports the ongoing negotiations with Iran. “We’re dealing with a country, Iran, who cannot be trusted. I would never trust Iran, everything that you do with Iran has to be verified.”

Apprehension toward Obama’s diplomatic approach toward Iran ranges from global and local issues. Rep. Dan Kildee (D-Mich.) is open to the landmark agreement itself, but he’s got concerns about constituent Amir Hekmati, a former Marine who is being held in Iran. In the private meeting, Kildee pressed the administration on the importance that Iran demonstrate that it isn’t trying to use Hekmati and fellow detainees Robert Levinson and Pastor Saeed Abedini as bargaining chips in the next step of the deal.

“No agreement to me that isn’t preceded by action by the Iranian government to release Amir Hekmati along with Mr. Levinson and Mr. Abedini can be taken as a credible agreement,” Kildee told reporters. “It is very difficult for me to imagine supporting or acknowledging the legitimacy of an agreement entered into with a nation who continues to hold one of my constituents — an innocent former united states marine — in an Iranian prison.”

The response from the Obama administration Wednesday was that the release of those prisoners is on a separate track from the broader nuclear deal, which involves other powerful nations that have less of an impetus to call for the release of Hekmati, Levinson and Abedini, Ruppersberger said.

The bipartisan legislation senators would likely devise opens a six-month negotiating window for Kerry before the new penalties take place, though it remains unclear if that bill will get a vote anytime soon.

But support for fresh economic penalties on Iran remains strong in the House, which voted 400-20 to limit Iran’s oil exports months before the breakthrough with Iran on a temporary nuclear deal.

Asked if he’d vote again today as he did in July to slap sanctions on Iran, Kildee said yes.

“The reason we’re in the position we are in right now, is that these sanctions are working,” he said.